Mix Design Optimization of Coal Gangue-Based Geopolymer Foamed Concrete Using Response Surface Methodology

This study develops a novel geopolymer foamed concrete using coal gangue and slag as precursors, along with a composite alkali activator comprising sodium silicate and sodium hydroxide, based on the physical foaming method. The Box–Behnken Design within Response Surface Methodology was applied to optimize the mix proportions of coal gangue–slag-based geopolymer foamed concrete. The effects of alkali activator dosage, sodium silicate modulus, water-to-binder ratio, and foam content on 28-day compressive strength and thermal conductivity were systematically investigated to determine the optimal mix for achieving a balance between mechanical and thermal performance. Scanning Electron Microscopy and other characterization techniques were used to analyze the microstructural features. The results show that foam content has the most significant influence on both mechanical and thermal performance, while the interaction between sodium silicate modulus and foam content exhibits the most pronounced combined effect. The optimized mix design consists of 9.1% alkali activator dosage, a sodium silicate modulus of 1.07, a water-to-binder ratio of 0.44, and foam content of 50%, resulting in a 28-day compressive strength of 2.30 MPa and thermal conductivity of 0.0781 W/(m·K). The observed performance enhancement is primarily attributed to the increased heterogeneity in the pore structure. This study provides theoretical and technical support for the development of integrated thermal insulation and load-bearing wall materials suitable for severely cold regions.